A. Field of the Invention
The invention relates to a densimeter and, more particularly to a densimeter which utilizes a float to sense the density of a fluid.
B. Prior Art
Float type densimeters generally incorporate a float having a net positive buoyancy with respect to any fluid in which it might be immersed. The float is maintained at a selected reference position by means of forces applied to it. The force required to so maintain it is a measure of the density difference between the fluid in which the float is immersed and the overall density of the float; as the fluid density changes, the force is changed correspondingly.
Commonly, the force is applied to the float magnetically by means of a permanent magnet or a soft iron core attached to the float and magnetically coupled to a current carrying coil adjacent the float. The coil generates a field which interacts with the magnet or iron core and exerts a force on it. As the fluid density changes, the driving current in the coil is varied to maintain the float in its reference position. The current in the coil is a measure of the force applied and thus of the density of the fluid. This current is sensed to provide the density-indicating output of the instrument.
Densimeters of this type have several diadvantages. To begin with, since the float always has a net positive buoyancy with respect to the fluid in which it is immersed, the minimum value of its buoyancy is determined by the minimum density of the fluids in which it is intended to be immersed. Since the densimeter is to be operable over a wide range of densities, restoring forces of substantial magnitude are often required to return the float to its reference position. This in turn can require large driving currents in the force coil. We have found that the need to measure the current over a wide range is a cause of substantial error in density determination. Moreover, a large current generates significant heat which causes a temperature rise in the fluid surrounding the coil and this can materially affect the measured density of the fluid.
In addition to the foregoing sources of error, the greater the float buoyancy which must be offset by the restoring force, the greater will be the error due to variations in gravity. These variations change the required restoring force by an amount proportional to the magnitude of the buoyancy and thereby cause an error in the same proportion. Thus, when sensors of this type are installed on a vessel which moves about from place to place, an undesirably large error is introduced simply from variations in the gravitational constant from place to place.
A small float buoyancy, and thus small float volume is also desirable for minimizing the force required to restore the float to its neutral buoyancy position. However, present densimeters, by requiring the float to carry the weight of the magnet or iron used in the force rebalancing system, limit the extent to which the volume of the float can be reduced and thus require larger restoring forces with the consequent disadvantages noted above.